It is usual for one or more vertical stabilisers to be positioned at the rear of an aircraft (see FIG. 1) and for a rudder R to be fixed to the rear of the fixed part V of the vertical stabiliser. The vertical stabiliser exists to provide lateral directional stability, i.e. to keep the aircraft nose pointing in the direction of flight and the rudder exists to provide lateral directional control, controlling the aircraft in yaw Y, for example in the case of engine failure, and to control the lateral direction of flight, for example during cross-wind landing.
Typical existing rudder mechanisms are shown in FIGS. 2a to 2c. The first mechanism shown in FIG. 2a comprises a fixed structure part V of a vertical stabiliser and a rudder R, or more generically a flight control member, hinged relative to the fixed structure of the vertical stabiliser by means of a simple hinge and the rudder is actuated by means of an actuator A. Most small piston-engined aircraft employ that arrangement.
FIG. 2b shows an off-set hinge arrangement where the hinge for hinging the rudder R to the fixed structure V is slightly off-set to one side of a notional centreline of the fixed structure but which still lies within the vertical stabliser outline. That allows the actuation mechanism also to be fitted within the outline of the vertical stabiliser so as to reduce drag.
Finally, FIG. 2c illustrates a double-hinged rudder arrangement. In that arrangement, the rudder R is formed in two parts R1, R2 which are articulated relative to each other and the rearmost part R2 of the rudder can be actuated relative to the main part of the rudder R1 so as to allow the rudder to be deflected to a greater angle than if the rudder was formed in one piece.
In addition, International Patent Publication No. WO97/32779 discloses a rudder and vertical stabiliser arrangement in which small flaps are formed just ahead of the rudder on the trailing edge of the vertical stabiliser. That arrangement can be used to open a path ahead of the rudder to allow air to flow ahead of the rudder.
European Patent Application Publication No. 0256374 discloses a split symmetrical rudder, each element being deflected independently relative to the fixed structure of the vertical stabiliser. In an extreme deployment both rudder parts can be deployed to effect air braking.
FIGS. 3a to 3e illustrate known trailing edge high lift devices, often referred to as flaps.
FIG. 3a shows a plain, simple hinged flap F on the trailing edge of a wing W. FIG. 3b shows a split flap in which only the lower surface of the trailing edge of the wing is hinged and the upper surface remains fixed in place. In FIG. 3c a slot S is created between the wing and the flap when the flap is deployed which improves airflow over the flap. In FIG. 3d a “Fowler” flap is shown. This arrangement is similar to the split flap except the hinged part of the flap translates rearwardly when deployed effectively increasing the area of the wing. Finally in FIG. 3e, a double-slotted Fowler flap is shown in which the hinged flap part F1 is translatable rearwardly relative to the wing and a second intermediate flap F2 is arranged between the leading edge of the main flap and the trailing edge of the wing. That further flap can be arranged so as to prevent or allow airflow from the underside of the wing to the upper side.
Various Fowler flap and slotted flap mechanisms are known. Flaps may be hinged relative to the wing by means of an off-set hinge, that provides some translation as the flap is deployed. A four-bar linkage may connect the flap to the wing, again providing translation as well as pivoting of the flap. The flap may run on a tailored track member allowing optimum flap travel and positioning for various flap positions.
When designing an aircraft the vertical stabiliser and the rudder are sized by various criteria, such as aircraft lateral stability, control of asymmetric yawing moments and aircraft lateral directional control. It is common for these criteria not to be balanced. Aircraft may require a considerably larger vertical stabiliser and rudder for the case of an engine out yawing moment than to provide normal lateral direction control. In such a case, an oversized stabiliser and rudder is provided which means that the aircraft is heavier and suffers from greater drag to account for the rare instance of an engine failure.
Conventional rudder mechanisms generally comprise a plain flap pivotable in either direction about a pivot point on a centreline of a fixed stabiliser structure. Slotted and Fowler flap designs are more aerodynamically effective for their size but existing slotted and Fowler flap mechanisms are uni-directional.
It is an object of the invention to provide an improved bi-directional flight control surface mechanism.